This application is based upon and claims the benefit of priority from prior Japanese Patent Applications P2001-85224 filed on Mar. 23, 2001; P2002-76334 filed on Mar. 19, 2002; P2002-76335 filed on Mar. 19, 2002; P2002-76226 filed on Mar. 19, 2002; P2002-76523 filed on Mar. 19, 2002; the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a multilayer wiring board assembly composed of a plurality of printed circuit boards in a multilayered structure, a multilayer wiring board assembly component for use in laminating the same and the manufacturing method of the same. In particular, the present invention relates to a multilayer wiring board assembly, a multilayer wiring board assembly component for use in laminating the same and the manufacturing method of the same, wherein the multilayer wiring board assembly is flexible and implemented with a highly packing density by the Flip Chip Mounting technique and so forth.
2. Description of the Related Art
A flexible printed circuit board (which is simply referred to as an xe2x80x9cFPCxe2x80x9d in this description) is composed of a resin film having a small thickness for the purpose of maintaining the flexibility thereof. Because of this, in the case of such FPCs, it is essentially very difficult to assemble a plurality of FPCs in a multilayered structure (a multilayer wiring board assembly). However, along with the advent of the implementation of FPCs with a highly packing density, the need for multilayered structures of FPCs has increased in recent years, for example, in consideration of the arrangement of lead wirings as connected to flip chips mounted on FPCs. In this situation, a multilayer wiring board assembly has been manufactured by multilayering a plurality of FPCs with an intervening glass epoxy prepreg sheet and so forth between each adjacent boards while one or both surface of each FPC has been formed with a circuit pattern, forming holes through the entirety of all the layers by means of a drill and the like, and interlayer interconnecting the layers by the use of a through hall plating and the like.
However, in the case of such a conventional method of manufacturing multilayer wiring board assemblies by the use of a through hall plating, it is impossible to form a via hole anew on another via hole and to mount a chip on a via hole, generally called as via-on-via since a hole remains in the center of a through hall even after plating. Because of this, when a multilayer wiring board assembly is implemented with a highly packing density, there are several impediments such that a lead can not be extended from the position just below the chip while interlayer interconnection tends to excessively occupy a substantial area.
On the other hand, for example, ALIVH (Any Layer Interstitial Via Hole: a registered trademark of Matsushita Electric Industrial Co., Ltd.) is a rigid multilayer wiring board assembly in which via-on-via is possible while a conductive paste is used for interlayer interconnection between each adjacent layers. An ALIVH board is formed by repeating the process sequence including opening a through hole in an uncured resin board, filling the through hole with a conductive paste, joining a copper foil to the uncured resin board, hardening the resin while compression bonding in order to form a multilayered structure and etching the copper foil in order to form a circuit pattern.
However, while via-on-via is possible in the case of the manufacturing method as described above to form ALIVH boards since the interlayer interconnection is made by means of a conductive paste, it is extremely difficult to manufacture FPCs in the form of a multilayer wiring board assembly by applying the manufacturing method to FPCs because it is necessary to open a hole through a resin film such as a polyimide film having a small thickness followed by filling up the hole with a conductive paste. This is because, when opening a hole in a resin film having a small thickness, the position and the size of the hole tend to change due to distortion of the resin film, the sucking force of a drill and the like so that the necessary accuracy of alignment is hardly achieved during the printing operation of the conductive paste and the positioning of the respective layers.
Also, while via-on-via is possible in the case of the manufacturing method making use of a conductive paste for the interlayer interconnection like ALIVH, it is difficult to make electric connection between a copper foil and a conductive paste without compromising the electric characteristics of the copper foil and the conductive paste so that the respective venders make use of proprietary methods respectively. Namely, generally speaking, when interlayer interconnection is made by via-on-via, a copper foil and a conductive paste are connected to each other by inserting the copper foil between adjacent conductive paste layers. In this case, the conductive paste is arranged to pierce the copper foil for the purpose of preserving electric connectivity between the copper foil and the conductive paste filler. For example, in the case of an ALIVH board, a conductive paste is printed in order to form a projection while the thickness of the board is reduced during thermocompression for bonding because of the use of an uncured resin board so that it becomes possible to make electric connection of a copper foil pierced by the projection of the conductive paste.
However, in the case of a board made of such a substance that the thickness of the board is not reduced during thermocompression for bonding, like a polyimide used for making a resin film in the case of FPCs, the projection of a conductive paste is less effective in piecing the copper foil. As a result, it is difficult to make electric connection between a copper foil and a conductive paste without compromising the electric characteristics.
Furthermore, in the case where a through hole is filled with a conductive paste, the conductive paste is printed in the condition that the surface of the conductive paste is slightly depressed since the conductive paste is pressed during printing. Because of this, there is a problem that, even if boards having through holes filled with a conductive paste are joined to each other, it is impossible to achieve sufficient electric connectivity between the fillers of the conductive paste.
The present invention has been made in order to solve the shortcomings as described above. It is an object of the present invention to provide a multilayer wiring board assembly, a multilayer wiring board assembly component and a method of manufacture thereof in which it is possible to easily laminate together flexible FPCs having highly packing densities by via-on-via and chip-on-via.
It is another object of the present invention to provide a multilayer wiring board assembly which can be manufactured by joining multilayer wiring board assembly components to each other with a uniform pressure and therefore it is possible to fixedly laminate the multilayer wiring board assembly with a better electric connectivity.
It is a further object of the present invention to provide a multilayer wiring board assembly which is manufactured in order that the opening of a through hole near a copper foil is placed on a plane table having a hole of a diameter larger than that of the through hole followed by filling the through hole with a conductive paste from a masking tape in order to form a brim laterally extending beyond the perimeter of the opening of the copper foil, and therefore it is possible to form the brim having a desired profile with a high degree of accuracy and having a smaller thickness than that in the case utilzing a mask, and therefore possible to fixedly laminate the multilayer wiring board assembly with a better electric connectivity.
It is a still further object of the present invention to provide a multilayer wiring board assembly in which the contact area of a conductive paste filler with a copper foil is increased when compression bonding multilayer wiring board assembly components to each other, and therefore it is possible to improve electric connectivity.
It is a still further object of the present invention to provide a multilayer wiring board assembly in which it is possible to manufacture a multilayer wiring board assembly having a good heat resisting property with little damage to the respective boards during the manufacturing process by the use of an adhesive layer made of a thermosetting resin.
It is a still further object of the present invention to provide a multilayer wiring board assembly in which it is possible to manufacture a multilayer wiring board assembly having a good heat resisting property without damaging the conductive resin components (the conductive paste) by the use of an adhesive layer formed of a thermoplastic resin to which is given a thermosetting property.
In accordance with an aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded by screen printing in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with another aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of forming a through hole in a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which said through hole is opened through said copper foil, said resin film and said adhesive layer; and a step of embedding a conductive paste filler by screen printing in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of etching a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof in order to form a predetermined circuit pattern; a step of forming a mask layer on said adhesive layer of the copper plated resin film having been formed with said circuit pattern; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said mask layer; a step of embedding a conductive paste filler by screen printing in the through hole of said copper plated resin film from said copper foil; and a step of removing said mask layer.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly component and a multilayer wiring board in accordance with the present invention, a through hole is opened through the copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface, and filled with a conductive paste, and therefore it becomes easy to form the through hole and embed the conductive paste because of the increase in thickness as compared with the case where a through hole is opened only through a resin film and filled with a conductive paste. In this case, it is possible to improve electric connectivity between the copper foil and the conductive paste filler by embedding the through hole in order that the conductive paste filler is laterally extending beyond the perimeter of the opening of the through hole.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is laminated with a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above in which the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with the copper foil or the conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is manufactured by laminating together a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein said multilayer wiring board assembly is laminated with a plurality of said multilayer wiring board assembly components through said adhesive layer in order that the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
Namely, in the case of the multilayer wiring board assembly and the manufacturing method thereof in accordance with the present invention, the conductive paste filler of the multilayer wiring board assembly component is embedded in the through hole of the copper plated resin film by screen printing with a leading end of the conductive paste filler being projected from the adhesive layer, and therefore it is possible to make reliable electric connection of the leading end of the conductive paste filler with the copper foil or the conductive paste filler of the multilayer wiring board assembly component and to improve electric connectivity between the respective layers when a plurality of the multilayer wiring board assembly components are laminated together. Particularly, in this case, direct connection between the conductive paste fillers without the intermediary copper foil is effective to improve the electric connectivity.
Meanwhile, the adhesive layer can be made of, for example, a thermoplastic adhesive. In this case, the respective adjacent multilayer wiring board assembly components are fixedly joined by means of the adhesive layer which is made soft by laminating the multilayer wiring board assembly components together and heating them, and therefore it is possible to make reliable electric connection of the conductive paste filler with the copper foil and the conductive paste filler of the adjacent multilayer wiring board assembly component. Also, in this case, smooth electrical connection can be achieved by finally curing the conductive paste filler at the same time as the multilayer wiring board assembly components are laminated together.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of forming a through hole in a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which said through hole is opened through said copper foil, said resin film and said adhesive layer; and a step of embedding a conductive paste filler in the through hole from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film with one end of said conductive paste filler being projected from said adhesive layer and the tail end of said conductive paste filler being projected to the same height as said copper foil.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer and with part of said conductive paste being laterally extending beyond the perimeter of the opening of the through hole of said copper foil.
In accordance with a further aspect of the present invention, a multilayer wiring board assembly comprises: a first multilayer wiring board assembly component comprising a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer, and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer and with part of said conductive paste being laterally extending beyond the perimeter of the opening of the through hole of said copper foil; and a second multilayer wiring board assembly component comprising a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer, and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer and the tail end of said conductive paste filler being projected to the same height as said copper foil,
wherein said second multilayer wiring board assembly component is located as the most outer layer of the multilayer wiring board assembly while said first multilayer wiring board assembly component is located as an inner layer of the multilayer wiring board assembly other than said second multilayer wiring board assembly, and wherein the leading end of said conductive paste filler of one of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
In accordance with a still further aspect of the present invention, the multilayer wiring board assembly is manufactured as described above and in order that said second multilayer wiring board assembly component is located as the most outer layer of the multilayer wiring board assembly while said first multilayer wiring board assembly component is located as an inner layer of the multilayer wiring board assembly other than said second multilayer wiring board assembly, and wherein wherein said multilayer wiring board assembly is laminated with a plurality of said multilayer wiring board assembly components through said adhesive layer in order that the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly component and a multilayer wiring board in accordance with the present invention, the upper surface of the most outer layer (most upper layer) is formed in order that the upper surface of the copper foil and the leading end of the conductive paste filler, i.e., the printing surface, are arranged to form a continuous flat surface having the same height so that it is possible to apply a uniform pressure to the entirety of the multilayer wiring board assembly during lamination.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of etching a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof in order to form a predetermined circuit pattern; a step of forming a mask layer on said adhesive layer of the copper plated resin film having been formed with said circuit pattern; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said mask layer; a step of placing, on said copper foil over said through hole, a mask of a thickness larger than that of said through hole; a step of embedding a conductive paste filler in said through hole with the tail end of said conductive paste filler being located at the same height as said mask; a step of removing said mask to leave said tail end of said conductive paste filler being projected from said copper foil with part of said conductive paste being laterally extending beyond the perimeter of the opening of the through hole of said copper foil; and a step of removing said mask to leave the tail end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises:a step of etching a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof in order to form a predetermined circuit pattern; a step of forming a mask layer on said adhesive layer of the copper plated resin film having been formed with said circuit pattern; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said mask layer; a step of placing said copper plated resin film on a plane table in which a hole is opened with a thickness larger than that of said through hole in order that said copper foil is located in a lower position with the through hole located over the hole of said plane table; a step of embedding a conductive paste filler in the through hole of said copper plated resin film from said mask layer with a leading end of said conductive paste filler being projected from said copper foil with part of said conductive paste being laterally extending beyond the perimeter of the opening of the through hole of said copper foil; and a step of removing said mask layer to leave the tail end of said conductive paste filler being projected from said adhesive layer.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly component in accordance with the present invention, the opening of a through hole near a copper foil is placed on a plane table having a hole of a diameter larger than that of the through hole followed by filling the through hole with a conductive paste from a masking tape in order to form a brim laterally extending beyond the perimeter of the opening of the copper foil, and therefore it is possible to form the brim having a desired profile with a high degree of accuracy and having a smaller thickness than that in the case utilzing a mask. By this configuration, the multilayer wiring board assembly is fixedly laminated with a better electric connectivity.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer and with the tail end of said conductive paste filler being projected from said copper foil.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is manufactured by laminating with a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein said multilayer wiring board assembly is laminated with a plurality of said multilayer wiring board assembly components through said adhesive layer in order that the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of preparing a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer bonded to the other surface thereof and forming first and second mask layers on said one surface and said other surface of said copper plated resin film; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said first and second mask layers; a step of embedding a conductive paste filler in said through hole with the leading and tail ends of said conductive paste filler being located at the same height as said mask layers; and a step of removing said first and second mask layers to leave the leading end of said conductive paste filler being projected from said adhesive layer with the tail end of said conductive paste filler being projected from said copper foil.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly component and a multilayer wiring board in accordance with the present invention, since the leading end of the conductive paste filler is projected from the adhesive layer while the tail end of said conductive paste filler is projected from said copper foil, the multilayer wiring board assembly is fixedly laminated with a better electric connectivity.
In accordance with a still further aspect of the present invention, multilayer wiring board assembly component comprises: a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermosetting resin and bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of forming a through hole in a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermosetting resin and bonded to the other surface thereof and in which said through hole is opened through said copper foil, said resin film and said adhesive layer; and a step of embedding a conductive paste filler in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of etching a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermosetting resin and bonded to the other surface thereof in order to form a predetermined circuit pattern; a step of forming a mask layer on said adhesive layer of the copper plated resin film having been formed with said circuit pattern; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said mask layer; a step of embedding a conductive paste filler in the through hole of said copper plated resin film from said copper foil; and a step of removing said mask layer.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is laminated with a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is manufactured by laminating together a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein said multilayer wiring board assembly is laminated through said adhesive layer in order that the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly component and a multilayer wiring board in accordance with the present invention, it is possible to manufacture a multilayer wiring board assembly having a good heat resisting property with little damage to the respective boards during the manufacturing process.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly component comprises: a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermosetting resin to which is given a thermosetting property and bonded to the other surface thereof and in which a through hole is opened through said copper foil, said resin film and said adhesive layer; and a conductive paste filler embedded in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of forming a through hole in a copper plated resin film made of a resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermoplastic resin to which is given a thermosetting property and bonded to the other surface thereof and in which said through hole is opened through said copper foil, said resin film and said adhesive layer; and a step of embedding a conductive paste filler in the through hole of said copper plated resin film from said copper foil with a leading end of said conductive paste filler being projected from said adhesive layer.
In accordance with a still further aspect of the present invention, a manufacturing method of a multilayer wiring board assembly component comprises: a step of etching a copper plated resin film made of a flexible resin film which is provided with a copper foil bonded to one surface thereof and an adhesive layer made of a thermoplastic resin to which is given a thermosetting property and bonded to the other surface thereof in order to form a predetermined circuit pattern; a step of forming a mask layer on said adhesive layer of the copper plated resin film having been formed with said circuit pattern; a step of forming a through hole opened through said copper foil, said resin film, said adhesive layer and said mask layer; a step of embedding a conductive paste filler by screen printing in the through hole of said copper plated resin film from said copper foil; and a step of removing said mask layer.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly laminated a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein the leading end of said conductive paste filler of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
In accordance with a still further aspect of the present invention, a multilayer wiring board assembly is manufactured by laminating together a plurality of multilayer wiring board assembly components including the multilayer wiring board assembly component as described above wherein said multilayer wiring board assembly is laminated with said adhesive layer in order that said leading end of said conductive paste filler of said at least one of said multilayer wiring board assembly components makes electric contact with said copper foil or said conductive paste filler of an adjacent one of said multilayer wiring board assembly components.
Namely, in the case of the manufacturing method of a multilayer wiring board assembly and a multilayer wiring board component in accordance with the present invention, by the use of an adhesive layer is formed of a thermoplastic resin to which is given a thermosetting property, and therefore it is possible to provide a multilayer wiring board assembly in which it is possible to manufacture a multilayer wiring board assembly having a good heat resisting property without damaging the conductive resin components (the conductive paste).